A known nondestructive testing method for detecting flaws which are present in a tubular metal body being inspected, for example a metal pipe which is used as an oil country tubular good (oil well tubing and casing), line pipe, or a mechanical part (such as a hollow shaft, mechanical tubing used in an automotive part, or a stainless steel pipe used in high temperature environments) without destroying it is the ultrasonic flaw detection method in which ultrasonic waves are impinged on a metal pipe and the reflected echoes from flaws present in its interior are detected. Among ultrasonic flaw detection methods, the angle beam ultrasonic flaw detection method in which ultrasonic waves are impinged on a surface undergoing flaw detection at an angle is used in order to detect flaws in the inner surface, in the outer surface, in the interior, and in welds of a metal pipe. As is well known, in the angle beam flaw detection method, normally, an angle probe is used which has an housing in which a transducer disposed so as to transmit ultrasonic waves at an angle with respect to a surface undergoing flaw detection, a sound absorbing material, and a couplant for contacting the surface undergoing flaw detection (a wedge or the like made of an acrylic or other resin) are included. In cases in which water is used as a couplant, instead of a wedge or other couplant being housed in a casing, flaw detection is carried out with the metal pipe and the angle probe immersed in water.
FIG. 11 is an explanatory view showing the relationship between incident waves 1 and refracted waves 2 and 3 in an angle beam flaw detection method. The dashed line in FIG. 11 and in FIGS. 12 and 13 to be described later indicates a normal to the flaw detection plane O.
As shown in FIG. 11, in the angle beam flaw detection method, when incident ultrasonic waves 1 are obliquely incident on the flaw detection surface O of a metal pipe (medium II), even in the case where the incident ultrasonic waves 1 emitted at an unillustrated transducer are longitudinal ultrasonic waves, refracted waves in the form of refracted longitudinal waves 2 and refracted transverse waves 3 are propagated inside the metal pipe. If the sound velocity of incident ultrasonic waves 1 in medium I (generally a liquid couplant typified by water or a wedge housed inside an angle probe) is Vi, the sound velocity of refracted transverse ultrasonic waves 3 in medium II (a metal pipe which is a tubular body being inspected) is Vs, the sound velocity of refracted longitudinal ultrasonic waves 2 in medium II is VL, the angle of incidence of incident waves 1 is θi, the angle of refraction of refracted transverse waves 3 is θs, and the angle of refraction of refracted longitudinal waves 2 is θL, then Snell's law, i.e., the relationship sin(θi/Vi)=sin(θs/Vs)=sin(θL/VL) is established between the incident waves 1 and the refracted waves 2 and 3.
FIG. 12 is an explanatory view showing the propagation of refracted waves 2 and 3 in the interior 5c of a metal pipe 5. As shown in this figure, if incident waves 1 from a transducer 4 of an ultrasonic probe are incident on the metal pipe 5 with an angle of incidence θi, refracted ultrasonic waves 2 and 3 are propagated in the interior 5c of the metal pipe 5 while repeatedly reflecting off the inner surface 5a and the outer surface 5b of the metal pipe 5. If a flaw is present on the inner surface 5a or the outer surface 5b or in the interior 5c of the metal pipe 5, a reflected echo of ultrasonic waves reflected from the flaw returns to the transducer 4 and is received as a flaw echo. In this manner, ultrasonic flaw detection of the metal pipe 5 is carried out.
As explained with respect to FIG. 11, refracted longitudinal waves 2 and refracted transverse waves 3 are both propagated in the interior 5c of the metal pipe 5, namely, in medium II, so it is difficult to distinguish whether an echo received by the transducer 4 is due to refracted longitudinal waves 2 or refracted transverse waves 3.
As a result, the location of a flaw cannot be specified, the wave shape of a received signal becomes complicated, and the S/N ratio of a flaw echo decreases.
Consequently, in general, in order to carry out ultrasonic flaw detection of a steel pipe 5 by the angle beam flaw detection method, the angle of incidence θi is set at an angle which is larger than the critical angle of the refracted longitudinal waves 2 so that refracted transverse waves 2 are not included in the refracted waves propagated in the interior 5c of the metal pipe 5. For example, when medium I is water, the sound velocity Vi of refracted longitudinal waves 2 in medium I at room temperature is approximately 1500 meters per second, and if the sound velocity VL of refracted longitudinal waves 2 in the metal pipe 5 which is medium II is 5900 meters per second and the sound velocity Vs of refracted transverse waves 3 is 3200 meters per second, then from Equation 1, the angle of incidence θi which becomes the critical angle of the refracted longitudinal waves 2 (θL=90°) becomes approximately 15°, and the angle of refraction θs of refracted transverse waves 3 becomes approximately 33°. Therefore, in principle, if the angle of incidence of θi of incident waves 1 is set to be at least 15°, only refracted transverse waves 3 are present in medium II.
In recent years, there has been an increasing demand not only for a reduction in weight but also an increase in strength of a steel pipe used as an oil country tubular good, line pipe, mechanical part, or the like. As a result, there is an increasing demand for a metal pipe having a large ratio (t/D) of the wall thickness t to the outer diameter D which is as high as at least 15%, for example (referred to in this specification as “high t/D metal pipes”). However, as shown in FIG. 13 which is an explanatory view of the situation when carrying out flaw detection on a high t/D metal pipe 6 by the angle beam flaw detection method, when angle beam flaw detection of a high t/D metal pipe 6 is carried out by the above-described conventional ultrasonic flaw detection method, even in case where waves are incident from the outer surface 6 of a high t/D metal pipe 6 with an angle of incidence θi of at least the critical angle of longitudinal ultrasonic waves of the ultrasonic waves 1, the refracted transverse waves 3 which are propagated in the interior 6c of the metal pipe 6 sometimes follow a propagation path to the outer surface 6b without reaching the inner surface 6a of the metal pipe 6. In this case, flaws present in the vicinity of the inner surface 6a of the metal pipe 6 cannot be detected.
Patent Document 1, for example, discloses using a first ultrasonic probe having a first transducer for which the refraction angle θs of refracted transverse waves inside a metal pipe is increased, such as to greater than 35°, and a second transducer for which the refraction angle θs is decreased, such as to less than 35°. The first transducer is used by itself when performing flaw detection of a metal pipe 5 having a usual ratio (t/D), and the first transducer and the second transducer are used together when performing flaw detection of a high t/D metal pipe 6.
If the ultrasonic probe disclosed in Patent Document 1 is used to perform flaw detection of a high t/D metal pipe 6, it is in fact possible for refracted transverse waves generated by the second transducer to reach the inner surface of the high t/D metal pipe 6. However, when the second transducer is used, not only refracted transverse waves but also refracted longitudinal waves are generated, so the position of a flaw can not be specified, the waveform of the received signal becomes complicated, or the S/N ratio of flaw echoes decreases.
Non-patent Document 1, for example, discloses an invention in which an acoustic lens having a front end surface with a spherical or cylindrical shape is disposed in front of a transducer, or in which the front end surface of the transducer is formed into a spherical or cylindrical shape, and when detecting flaws which are short in the axial direction of a metal pipe and have a small depth, an acoustic lens having a spherical end surface or a probe formed to have a spherical end surface is used, and when detecting flaws which are shallow but continuous in the pipe axial direction, an acoustic lens having a cylindrical end surface or a transducer which is formed to have a cylindrical end surface with the direction of curvature of the cylindrical surface extending in the circumferential direction of the metal pipe is used, whereby ultrasonic waves incident on the metal pipes are focused onto the metal pipe, and as a result, the strength of echoes is increased, whereby detection can be performed with a good S/N ratio and minute flaws formed in the interior of a metal pipe can be detected with high accuracy.
FIG. 14 is an explanatory view showing the propagation of refracted longitudinal waves 2 and refracted transverse waves 3 which are propagated in the interior of metal pipes 5 and 6 when refracted transverse waves 3 are focused on the inner surface of the metal pipes 5 and 6 in accordance with the invention disclosed in Non-patent Document 1. FIG. 14(a) shows refracted transverse waves 3 when using a high t/D metal pipe 6 for which the ratio (t/D) is at least approximately 15%, FIG. 14(b) shows refracted longitudinal waves 2 when using this high t/D metal pipe 6, FIG. 14(c) shows refracted transverse waves 3 when using a metal pipe 5 for which the ratio (t/D) is less than approximately 15% (around 10%), and FIG. 14(d) shows the case when using this metal pipe 5.
As shown in FIG. 14(c) and FIG. 14(d), in the case of a usual metal pipe 5 for which the ratio (t/D) is less than approximately 15%, ultrasonic flaw detection can be carried out by easily establishing conditions such that refracted transverse waves 3 are focused on the inner surface 5a of the metal pipe 5 and refracted longitudinal waves 2 are not generated. In contrast, as shown in FIG. 14(a) and FIG. 14(b), in the case of a high t/D metal pipe 6 for which the ratio (t/D) is at least approximately 15%, if it is attempted to make refracted transverse waves 3 reach the inner surface 6a of the metal pipe 6, refracted longitudinal waves 2 are also produced.
A portion of the refracted longitudinal waves 2 which are generated reach the inner surface 6a of the metal pipe 6 in the same manner as the refracted transverse waves 3, and the arriving refracted longitudinal waves 2 are propagated at an angle which is close to perpendicular with respect to the inner surface 6a of the metal pipe 6. As a result, they are reflected multiple times between the inner surface 6a and the outer surface 6b of the metal pipe 6.
FIG. 15 is a graph showing one example of reflected echoes observed when performing flaw detection of a high t/D metal pipe 6 in this manner. As illustrated by the graph in FIG. 15, an echo from an inner surface flaw by refracted transverse waves 3 is buried among the multiply reflected echo of the refracted longitudinal waves 2. This multiply reflected echo of the refracted longitudinal waves 2 becomes a noise signal which interferes with detection of flaws, and minute flaws cannot be detected with a high S/N ratio. Depending on the wall thickness of the metal pipe 6, a flaw echo is completely buried in the flood of multiply reflected echoes of the refracted longitudinal waves 2, and even an experienced inspector cannot distinguish flaw echoes.
Patent Document 2 discloses an invention in which flaw echoes in a high t/D metal pipe are found by alternatingly performing flaw detection at two frequencies in which flaw echoes and multiply reflected echoes are detected by flaw detection at a certain frequency and only multiply reflected echoes are detected by flaw detection at a different frequency, and the multiply reflected echoes which are noise are removed by differential processing of the flaw detection waveforms at these frequencies.
Patent Document 1: JP 10-90239 A1 (1998)
Patent Document 2: JP 06-337263 A1 (1994)
Non-patent Document 1: “Ultrasonic Flaw Detection”, Japan Society for the Promotion of Science, 19th Steelmaking Committee, published by Nikkan Kogyo Shimbun, Ltd., pp. 224-227.